Mmmm, beefy.

Blizzard has beefed up the blood elves in an outcry from the community that said that the male blood elves looked too feminine.

As many players have noticed, we have made a slight change to the male Blood Elf character models. In response to concerns that the Blood Elf male appeared to be too feminine, and after reviewing the model from a visual and conceptual standpoint, the decision was made to increase the body mass to give them a more substantial, masculine feel. It was also important that as members of the Horde that the Blood Elves gave the impression of strength and a more menacing presence.

GU Comics provides an alternate explanation for the suddenly buffed up elves.

View the comic here.
Also, you can read the post made by Blizzard poster Nethaera regarding the change on the official beta forums.